1. Field of the Invention
The present invention relates to an LED (light-emitting diode) light emitting device. Preferably, the present invention relates to an LED light emitting device with high color rendering index (hereinafter, CRI) and high luminescence efficiency.
2. Description of Related Art
In recent years, blue light LEDs are often combined with fluorescent materials to produce light emitting devices which can emit white light. Such light emitting devices are often used as backlight sources of LCD monitors, traffic signals, illuminated switches, or indicators. Moreover, since the LEDs taken as light sources in the light emitting devices only require a low current to operate, the light emitting devices can considerably reduce the energy consumed in comparison with conventional incandescent lamps or fluorescent lamps. In addition, light emitting devices using LEDs as light sources can have a longer service life than that of conventional incandescent lamps or fluorescent lamps.
FIG. 1 is a schematic diagram showing a conventional light emitting device employing a combination of a blue light LED and a fluorescent material. A blue light LED 102 is disposed on a substrate 101, and an input terminal 105 is connected to the blue light LED 102 for providing electric energy to the light emitting device. The blue light LED 102 is covered with a layer of fluorescent material coating 103 (yttrium aluminum garnet, YAG). Furthermore, the light emitting device is covered with a transparent hemispherical enclosure 104 to protect the blue light LED 102 and the fluorescent material coating 103 therein from the influence of moisture. However, such light emitting device lacks a red waveband (wavelength ranging from 600 nm to 650 nm approximately), so that the CRI of the emitted light is low. It should be understood that, as is well known and accepted, CRI is a relative difference between the color of a specimen under the illumination of a light emitting device and the color of the specimen under the illumination of a black-body radiator. When CRI is 100, the color of the specimen under the illumination of the light emitting device is substantially the same as the color of the specimen under the illumination of the black-body radiator.
Referring back to the light emitting device in FIG. 1, it is assumed that the fluorescent material coating 103 employs a mixed fluorescent material containing fluorescent materials capable of being excited by the blue light LED 102 and respectively emitting red light and green light. Although the mixed fluorescent material can improve the CRI of the light emitting device, the green light excited by the blue light LED 102 is reabsorbed by the material capable of emitting red fluorescent light, which causes the luminescence efficiency (i.e. lumens per watt, lumens/W) of green light to decrease, and thus the luminescence efficiency of the entire light emitting device is affected.
For the problem of the low CRI of the conventional light emitting device, an LED capable of emitting light with a dominant wavelength in a range between 430 nm and 480 nm is covered with a fluorescent material (Cerium-doped YAG, YAG:Ce) capable of emitting light with a dominant wavelength ranging from 555 nm to 585 nm, while an LED capable of emitting light with a dominant wavelength ranging from 600 nm to 630 nm is added to the light emitting device. Therefore, a red waveband lacking in the conventional light emitting device can be supplemented, thereby increasing the CRI of the emitted light to 80. However, the luminescence efficiency of the light emitting device is still low (refer to Table 1).